CMOS integrated circuitry is widely used in modern electronic applications. Such circuitry has been a significant factor in the low cost construction of highly sophisticated systems, and particularly is of significance in popular personal computer systems currently in widespread use. These systems, and many other electronic systems used in a variety of applications, employ binary digital signals to effect the various operations in the systems. Because of the high speeds employed in such binary digital signals, the fast rise and fall times of the signals can cause interference with the operations of other portions of a circuit within a machine or system. Radiation of undesirable signal spikes outside of a system also may cause interference with other electronic apparatus operating nearby.
In the past, the solutions for eliminating the effects of electro-magnetic interference radiation (emi) ranged from shielding circuit areas which were subject to generating such interference to the use of ferrite beads on circuit leads, where such signals appeared, to round the edges of the signals; so that the unwanted spikes did not occur. These solutions require additional physical apparatus to accomplish them. As a result, additional bulk is built into any system which requires such shielding or ferrite beads; and additional manufacturing complexities are created.
A specific circuit area for personal computers which has emi problems is the keyboard clock lines, due to the fast fall times of the clock signals. Due to the slow speed requirements on the keyboard to system interface, it is not necessary to have fast fall times. The typical CMOS bonding pad drive circuit, however, which has been used in the past, is configured to rapidly switch the respective P-channel and N-channel transistors on and off quickly in response to input signals, thereby transferring fast rise and fast fall times to the output bonding pad driven by such a circuit.
It is desirable to eliminate or minimize emi problems on a CMOS circuit output pad while providing high current drive capability without requiring external shielding or ferrite beads to accomplish the desired result.